At 5:04 PM on October 17, 1989, the San Francisco Bay Area was severely shaken for 15 seconds by the Loma Prieta earthquake located to the south on the San Andreas fault near Santa Cruz. Although the quake did not tear the ground surface, it collapsed some buildings and freeway overpasses built upon the soft "bayfill" sediment in San Francisco and Oakland. A section of the Bay Bridge collapsed. .
The epicenter of the magnitude 7.1 quake was located about 10 miles northeast of Santa Cruz along a segment of the San Andreas fault, near Loma Prieta peak, the highest peak in the Santa Cruz Mountains. It was felt over an area of about 54,000 square miles. The rupture occurred on a 30-mile length of the San Andreas fault called the Santa Cruz Mountains segment. The focus was eleven and a half miles underground at a spot near China Ridge in Nisene Marks State Park. The land on the seaward side of the fault slipped five and a half feet northwest. .
The Loma Prieta quake was the largest earthquake to occur in the San Francisco Bay Area since 1906, and the largest, at the time, anywhere in California since 1952. The earthquake was responsible for 67 deaths, 3,757 injuries, and about 7 billion dollars worth of damage, making it the second biggest dollar loss natural disaster in the United States history. There were 1,018 homes destroyed and 366 businesses destroyed. Some 23,408 homes were damaged and 3,530 businesses damaged. .
A section of the San Francisco-Oakland Bay Bridge had collapsed. The top section of the Cypress viaduct of Interstate Highway 880 had collapsed on the lower section. The Marina District was in flames and several buildings had collapsed due to liquefaction. .
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The Loma Prieta quake was not the "Big One", but it was too big for comfort. 5 This rupture has not altered the assessment that there is a 50 percent chance for one or more magnitude 7.0 earthquakes in the San Francisco Bay Area in the next 30 years.
